(1) The CD4 molecule of T helper lymphocytes binds the envelope gp120 of HIV and thereby mediates viral entry. CD4 is also present in the plasma membrane of the endothelial and Kupffer cells lining the sinusoids of the human liver.In the present studies using immunoperoxidase staining, we found a similar localization of CD4 in chimpanzee liver. In addition, by treating sections with gp120 followed by an antibody to gp120, we show that hepatic sinusoidal CD4 effectively binds gp120. These results implicate the hepatic sinusoidal endothelial cell as an important virus reservoir in HIV infection. (2) Studies of the structure and function of the inner medulary collecting duct (IMCD) portion of the mammalian nephron have been difficult because of its inaccessibility. In the present studies individual rat IMCDs are dissected, mounted on glass cannulas and perfused. After physiological measurements have have been made the tubule is fixed and prepared for electron microscopy. Preliminary results have shown that the in vitro urea and water transport of IMCDS are responsive to the hormone vasopressin and that cell structure is well maintained By this approach we hope to gain insight into these functions which are the basis of the regulation of water homeostasis in the organism. (3) Pneumolysisn (ply) is a toxin produced by many strains of pneumococcal bacteria and plays an important role in their pathogenesis. In these studies rabbit antiserum prepared against E.coli-cloned ply was used to examine the expression of ply in 38 pneumococcal strains. In 4 of these strains ply was localized by electron microscopy in whole fixed cells using the ply antibody and a gold-labeled secondary antibody. Gold labelling was exclusively intracellular and was associated with various-sized clumps of amorphous material closely adherent to the cell membrane. No sequestering membranes or special transport structures were found; release occurs by cell lysis.